Crystal engineering:: Identification of a unique supramolecular synthon based on C=O•••X interaction in halogen-substituted aromatic carboxaldehydes

The well-known C=O...X interactions control the crystal packing of o-haloaromatic carboxaldehydes as revealed by the X-ray crystal structures of a set of aldehydes 2-8. The intramolecular C-H...X interaction causes the CO...X interactions to manifest in a readily recognizable pattern of association, i.e., synthon 1. The robustness of synthon 1 is amply suggested from the isostructurality exhibited by the dichloro- and dibromo-dialdehydes 3,6 and 4,7. Further, it is found that the aldehyde 2, which can exploit the C=O...Br interactions in two distinct ways crystallizes in two crystal modifications.